This invention relates in general to the monitoring of radio frequency (RF) high power radio systems and more specifically to the monitoring of a transmitter combiner radio system and associated antenna network.
Many types of two-way radio system configurations are used today for all types of purposes. Radio systems can be either private or public and can be utilized for applications in public safety, law enforcement, private dispatch and military. Often these systems are very complex and require some type of monitoring system to continually test and evaluate system performance in order to alert the system operator in the event of a pending failure. The most common failures in this type of complex radio system occur when a transmitter combiner and one of its cavity bandpass filters is out of tolerance or the antenna network degrades.
When these devices are out of tolerance, the systems effective radiated power is reduced thus the coverage on that channel or group of channels degraded. A de-tuned combiner cavity is a typical example of system degradation where a portion of the forward power is reflected back to the isolator, which dissipates the energy in the RF load and only a small fraction of that reflected power is passed back to the combiners input port too small for the fault to be detected by the transmitter. The transmitters directional coupler cannot detect this problem until the isolator is degraded (isolator is heat damaged and will need replacement) to the point where a significant amount of energy is passed back out the combiner""s input port. A directional coupler on the combiner output will not detect even substantial reductions in output power on a single channel with respect to the total output power of the combiner and due to the random nature of the channel activity.
The current technique of inserting a directional coupler onto the input and output ports of a transmitter combiner is ineffective as stated previously. A directional coupler on the combiners input port duplicates the repeater""s built in power monitor, cannot detect any problem past the combiner""s isolator, has insertion loss, and creates additional points of failure. The input port coupler can only provide additional information in the unlikely condition where the coax becomes lossy without reflected power to the transmitter.
A directional coupler on the combiner""s output port typically adds approximately 0.6 dB of loss to the transmit network and is very limited in its diagnostic abilities. The forward power out of the combiner is continually changing as the number of keyed transmitters varies randomly. This requires a summing and comparison logic circuit to adjust the forward power alarm threshold to be anywhere close to accurate, which is rarely done. Normally the alarm thresholds are set substantially high to avoid false alarms that it will only detect catastrophic failures where the combiner""s insertion loss approaches 50%. The same holds true for the reflected power threshold or it needs to be set substantially high to accommodate with all transmitters keyed condition.
With the reflection threshold set that high to avoid false alarms, the directional coupler may only indicate a problem when most of the stations are keyed or with a catastrophic failure of the antenna system. The users normally notice the drastic reduction in coverage due to antenna failure and call for service thus minimizing the effectiveness of directional couplers to monitor transmitter combiners and associated antenna network.
Thus, there is a need to provide a monitoring system that can detect the most probable failure in a transmitter combiner and antenna network before additional damage of an isolator or other associated equipment can occur. With transmitter combiners and antenna systems being the weakest links in today""s high power radio systems, and with higher system reliability required, the need exists to provide an adequate monitoring system to evaluate the relative performance and efficiency of those subsystems. Standard approaches used in the field either duplicate an existing function or they provide unusable readings during normal system operation. Existing monitoring methods cause serious problems since the insertion losses associated with the directional coupler reduces the performance of the radio system. Moreover, power detectors in the RF transmit path give rise the increased probability of component failure and IM (Inter-Modulation) production thus lowering the system reliability and performance. Hence, some additional system and method is required to avoid these inherent problems in the generally accepted testing methods.